It is known in the prior art to inspect an object by illuminating it with penetrating radiation, such as x-rays, for example. Some of the radiation may pass through the object, and some may be absorbed or scattered by the object. Of incident radiation scattered in all directions, scattered radiation back in the general direction from which it was incident may be referred to as backscatter radiation. Such scattered radiation may pass into a detector (which may be referred to herein as a “scatter detector”), and some portion of that scattered radiation will be detected by the scatter detector.
Scattered radiation highlights features, including concealed contraband or plastic explosives, that are characterized by a lower atomic number, whereas penetrating radiation is more likely to be absorbed in the bulk of higher-atomic number material, since photo-electric absorption increases very rapidly (as between the fourth and fifth power) with atomic number. The metallic body of an aircraft will highly absorb x-rays, whereas contraband materials will be more evident in x-ray backscatter.
Since, for a fixed detection efficiency per unit area, the signal-to-noise ratio increases with solid angle subtended by the scatter detector, backscatter systems tend to be ponderous and not well suited to inspection within confined area. This has had the effect of limiting the applicability of backscatter inspection modalities.